Center LaTeX and figures in markdown

This is to address #3224.  I have designed this so that the equations in Markdown cells match the indentation level of those in output.  This creates the desired left margin in a consistent way.  Here is what it looks like:

![screen shot 2013-08-31 at 5 40 19 pm](https://f.cloud.github.com/assets/27600/1063283/cff317fc-129f-11e3-9bb2-40cf5832ac5c.png)